Vacuum type electric circuit interrupting devices

ABSTRACT

A vacuum interrupter having two contacts with co-operating contact-making parts each of which is constituted by a porous matrix of metal particles comprising chromium containing from 0.5 percent to 13.5 percent by weight carbon, or iron containing from 1.0 percent to 2.0 percent by weight carbon, metallurgically bonded together by compacting and heating under high vacuum, the interstices of the matrix being infiltrated under high vacuum with a metal which comprises copper or a copper alloy, and the infiltrated metal constituting between 10 percent and 40 percent of the volume of the infiltrated matrix. When the matrix metal is chromium the preferred range of the carbon content is 1 percent to 3 percent by weight of the matrix metal.

United States Patent Wood June 28, 1974 VACUUM TYPE ELECTRIC CIRCUITPrimary Examin er-Robert S. Macon INTERRUPTING DEVICES Attorney, Agent,or Firml(irschstein, Kirschstein, Ot- [75] Inventor: Allan John Wood,Weeping Cross, tmger & Frank England [73] Assignee: The English ElectricCompany [57] Limited, London, England A vacuum interrupter having twocontacts with co- [22] Filed; May 1973 operating contact-making partseach of which is constituted by a porous matrix of metal particlescompris- FN- 3 ing chromium containing from 0.5 percent to 13.5 percentby weight carbon, or iron containing from 1.0

percent to 2.0 percent by weight carbon, metallurgi- [52] US. Cl 200/144B, 200/166 C cally bonded together by compacting and heating [51] Int.,Cl. HOlh 33/66 under high vacuum, the nt rsti es of th matrix eing [58]Field of Search 200/144 B, 166 C infiltrated under high vacuum with ametal which comprises copper or a copper alloy, andthe infiltrated metalconstituting between 10 percent and 40 percent of the volume of theinfiltrated matrix. When the ma- [56] References Cited Lrix metal ischromium the greferred rgnge of hthefcalron content is percent to ercenty we] to t e UNITED STATES PATENTS matrix metaL P g 2,975,256 3/l96l Leeet al. 200/144 B 355L622 12/1970 Takeuchi 200/166 C X 7 Claims, 2Drawing Figures ingoccurs between contacts when they are separated tointerrupt current, the mechanism of arc extinction in vacuuminterrupters is somewhat different from the mechanism of arc extinctionin other types of circuit interrupters in which the arcing occurs in amedium such as insulating gas or oil, and the tendency for the contactsto weld together is very much more severe. An object of this inventionis to provide a vacuum interrupter having contacts with good anti-weldproperties.

According to the present invention a vacuum interrupter comprises twocontacts having co-operating contact-making parts each of which isconstituted by a porous matrix of metal particles comprising chromiumcontaining from 0.5- percent to 13.5 percent by weight carbon, or ironcontaining from 1.0 percent to 2.0 per-- cent by weight carbon,metallurgically bonded together by compacting and heating under highvacuum, the interstices of the matrix being infiltrated under highvacuum with a metal which comprises copper or a copper alloy, and theinfiltrated metal constituting between percent and 40 percent of thevolume of the infiltrated matrix.

When the matrix metal is chromium the preferred range of the carboncontent is from 1 percent to 3 percent by weight of the matrix metal.

Preferably the matrix metal comprises a matrix of metal particlessintered together, the metal particles being of size not exceeding 250microns. The infiltrated metal may comprise an alloy of copper andsilver.

Alloys of copper suitable for the infiltrated metal may also includezirconium, tantalum or titanium, though only in small proportions; forexample, the copper, alloy may consist of 99.7 percent copper and 0.3percent zirconium by weight.

' The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIG. 1 is a vertical cross-section through a vacuum interrupterembodying the invention; and

FIG. 2 shows diagrammatically and to a very much larger scale averticalcross-section through an arcing portion of one contact of thevacuum interrupter shown in FIG. 1.

Referring to FIG. 1, the vacuum interrupter comprises a pair of endplates 11, 12 bonded in a vacuumtight manner respectively to cylinders13, 14 of insulating material. The cylinders 13, '14 are bonded to aflange 15 which is trapped between them, and carries a shield 16 ofgenerally cylindrical form. 3

The vacuum circuit interrupter is provided with a pair of relativelyseparable contacts 17, 18, the movable contact 17 being capable ofmovement by means of an actuator (not shown) towards and away from thefixed contact 18. The movable contact 17 has its contact stem 21reciprocable in a bushing 19, and a flexible conductor is provided whichis attached to the contact stem 21. A bellows device 20 is secured in avacuum-tight manner to the contact stem 21 and to the base plate 12 toallow movement of the contact 17.

The contact stem 21 has a contact head 22 secured -to it. The latter isrecessed at its centre to afford a flat annular face 23, whichco-operates with a similar face 24 on the co-operating contact 18 whenthe contacts are moved into engagement. The fixed contact 118 has a stem26, which is secured to the base plate 1111 and provides the otherterminal of the circuit interrupter, and a head 27 which mayconveniently be symmetrical with that of the movable contact 17.

In the case of a vacuum spark gap, the arrangement could be exactly asillustrated except that the elec' trodes would always be spaced apartand would not move relative to one-another, but nevertheless an arc maybe drawn between the faces 23, 24.

The contact heads 22, 27 are manufactured by compacting commerciallyavailable chromium powder e.g. powder made by the aluminothermicprocess) of particle size not exceeding 250 microns, to which sufficientcarbon has been added to bring the carbon content within the preferredrange of 1 percent to 3 percent by weight, and then sintering thecompact under high vacuum. In the normal aluminothermic process formaking commercially available chromium powder a small amount of carbonenters the chromium from the reactants, usually less than 1 percent byweight,-so that the addition of carbon is necessary to obtain therequired carbon content. Also there is a carbon reduction method for theproduction of chromium, in which the metal is reduced from the oxide bycarbon in an electric furnace. Carbon can readily be added to yieldchromium containing carbon within the preferred range of 1 percent to 3percent by weight. The sintered compact is then infiltrated with moltencopper under high vacuum and at a high temperature.

The copper occupies between 10 percent and 40 percent of the volume ofthe infiltrated matrix material, as determined by the porosity of thesintered compact, and hence as determined by the degree of compactionapplied to the compact. If necessary, the infiltrated compact may beshaped by normal machining methods. The chromium powder on compactionyields a matrix metal of low ductility, so that the infiltrated matrixlikewise exhibits a low ductility.

In place of chromium containing from 0.5 percent to l3.5 percent byweight carbon, the metal matrix may comprise iron containing from 1.0percent to 2.0 percent by weight carbon. Moreover, the infiltrated metalmay comprise alloys of copper with another metal of good electricalconductivity, for example silver.

Alloys of copper suitable for the infiltrated metal may also includezirconium, tantalum or titanium,

though only in small proportions; for example, the alloy may'comprise99.7 percent copper and 0.3 percent zirconium by weight.

In FIG. 2 a typical microstructure of the contact material is shown, thescale marking representing 200 microns, and each chromium-carbonparticle 30 is arrached to the neighbouring particles of chromiumvacuuminterrupter it is believed that microdeformation occurs, resulting in alarge number of well distributed contact points. It is considered thatthis condition would tend to result in low constriction resistance, gooddistribution of the FR loss at high current flow immediately prior toarcing, and little tendency to weld.

When such vacuum interrupter contacts breakwhich is effected at highvelocity by means of well known per seit is thought that there is a highprobability of multiple arcs being formed between the faces 23, 24,giving good-distribution of the arc energy and consequently low anduniform erosion.

Even after arcing, it is thought that the size of asperities tends to belimited to that of the maximum dimensions of the particles of thematrix, so that the local electric field and consequent field emissionfor a given contact separation is low, and the breakdown voltage ishigher and more predictable than for contacts of similar shapes mademainly of ductile materials.

Moreover, since the boiling point of the matrix material is below3,000C, electron emission densities following a high-current arcing loopare reduced by several orders below that which occurs with a tungstenmatrix when surface melting occurs, thus allowing a substantialimprovement to be achieved in the recovery voltage performance.

Tests on contacts made in accordance with the invention fromchromium-carbon compacts impregnated with 33 percent copper by volumehave shown that arcs up to at least kA peak can be interruptedsatisfactorily with low'and uniform contact erosion and no detectablewelding prior to arcing.

As stated above the chromium-carbon constituent of the contact materialmay be replaced by iron with the required carbon content. The mainproperties of each such matrix are:

a. it is capable of being wetted by the impregnating metal duringinfiltration;

b. its melting point is higher than that of copper, and is preferablyover l,200C, so that it is not melted by the infiltration process;

0. its melting point is lower than that of molybdenum;

d. its boiling point is not substantially greater than 3,000C; and e. ithas a low ductility in relation to that of copper.

As stated above the copper constituent of the contact material may bereplaced by a suitable copper alloy, for example an alloy of copper andsilver.

The main properties of each such infiltration metal are:

l. it has high electrical conductivity in relation to that of the matrixmetal; 2. its melting point is below that of the matrix metal,

and is below 1,200C;

3. its boiling point is not substantially greater than. and ispreferably below that of the matrix metal; 4. it has high ductilityrelative to that of the matrix metal; and 5. it has low viscosity whenmolten so as to facilitate infiltration. Furthermore, the addition ofcarbon to the matrix metal in the specified amounts results in carbonprecipitating in the grain boundaries, introducing weak links which aremaintained in the particulate metal and in the final sintered compact.Since the breaking of a weld is initiated by the rupture of the matrixmetal rather than the more ductile infiltrated metal, the presence ofthe carbon insignificantly improves the anti-weld properties of theelectrodes.

1 claim:

1. A vacuum interrupter comprising two contacts having co-operatingcontact-making parts each of which is constituted by a porous matrix ofmetal particles comprising chromium containing from 0.5 percent to 13.5percent by weight carbon, or iron containing from 1.0 percent to 2.0percent by weight carbon, metallurgically bonded together by compactingand heating under high vacuum, the interstices of the matrix beinginfiltrated under high vacuum with a metal which comprises copper or acopper alloy, and the infiltrated metal constituting between 10 percentand 40 percent of the volume of the infiltrated matrix.

2. A vacuum interrupter as claimed in claim 1, wherein the matrix metalis chromium and the range of the carbon content is from 1.0 percent to3.0 percent by weight of the matrix metal.

3. A vacuum interrupter as claimed in claim 1, wherein the particle sizeof the matrix particles throughout the matrix does not exceed 250microns.

4. A vacuum interrupter as claimed in claim 1. wherein the infiltratedmetal occupies 33 percent ofthe volume of the infiltrated matrix.

5. A vacuum interrupter as claimed in claim 1, wherein the infiltratedmetal comprises an alloy of copper and silver.

6. A vacuum interrupter as claimed in claim 1, wherein the infiltratedmetal comprises an alloy of copper with a metal chosen from the groupzirconium, tantalum and titanium.

7. A vacuum interrupter as claimed in claim 1, wherein the infiltratedmetal comprises an alloy consisting substantially of 99.7 percent copperand 0.3 percent zirconium by weight.

2. A vacuum interrupter as claimed in claim 1, wherein the matrix metalis chromium and the range of the carbon content is from 1.0 percent to3.0 percent by weight of the matrix metal.
 3. A vacuum interrupter asclaimed in claim 1, wherein the particle size of the matrix particlesthroughout the matrix does not exceed 250 microns.
 4. A vacuuminterrupter as claimed in claim 1, wherein the infiltrated metaloccupies 33 percent of the volume of the infiltrated matrix.
 5. A vacuuminterrupter as claimed in claim 1, wherein the infiltrated metalcomprises an alloy of copper and silver.
 6. A vacuum interrupter asclaimed in claim 1, wherein the infiltrated metal comprises an alloy ofcopper with a metal chosen from the group zirconium, tantalum andtitanium.
 7. A vacuum interrupter as claimed in claim 1, wherein theinfiltrated metal comprises an alloy consisting substantially of 99.7percent copper and 0.3 percent zirconium by weight.